In process automation technology, field devices are often applied, which serve to register and/or influence process variables. To register process variables, sensors serve as, for example, fill level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, or conductivity. Serving to influence process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, or the fill level in a container can be changed. In principle, all devices, which are applied near to the process and which deliver or process the process relevant information, are referred to as field devices. Besides sensors and actuators, generally also referred to as field devices are any units, which are directly connected to a fieldbus and which serve for communication with superordinated units, e.g. as remote I/Os, gateways, linking devices, and wireless adapters. A multiplicity of such field devices are produced and sold by the Endress+Hauser group.
In modern industrial plants, field devices are, as a rule, connected with superordinated units via bus systems (Profibus®, Foundation® Fieldbus, HART®, etc.). Normally, superordinated units involve control systems or control units, such as, for example, a PLC (programmable logic controller). The superordinated units serve, among other things, for process control, process visualization, process monitoring as well as start-up of the field devices. The measured values registered by the field devices, especially sensors, are transmitted via the connected bus system to one or, in given cases, also to a number of superordinated units. Along with that, a data transmission from the superordinated unit via the bus system to the field devices is also required; such data transmission serves especially for configuring and parametering field devices or for diagnostic purposes. Generally stated, the field device is serviced from the superordinated unit via the bus system.
Besides hardwired data transmission between the field devices and the superordinated unit, the possibility of wireless data transmission also exists. Especially in the bus systems Profibus®, Foundation® Fieldbus and HART®, wireless data transmission via radio is provided for by specification. Additionally, radio networks for sensors are specified in the standard IEEE 802.15.4 in greater detail. For implementing wireless data transmission, newer field devices, especially sensors and actuators, are, in part, embodied as radio field devices. These have, as a rule, a radio unit and an electrical current source as integral components. In such case, the radio unit and the electrical current source can be provided in the field device itself or in a radio module durably connected to the field device. Through the electrical current source, an autarkic energy supply is enabled in the field device.
Besides this, there is the opportunity to turn field devices without radio units into radio field devices, by coupling with a wireless adapter, which has a radio unit. A corresponding wireless adapter is described, for example, in the International Publication WO 2005/103851 A1. The wireless adapter, as a rule, is releasably connected to a fieldbus communication interface of the field device. Via the fieldbus communication interface, the field device can transmit the data to be transferred via the bus system to the wireless adapter, which then transmits these via radio to the target location. Conversely, the wireless adapter can receive data via radio and forward it via the fieldbus communication interface on the field device. Supplying the field device with electrical power occurs then, as a rule, via an energy supply unit of the wireless adapter.
In the case of autarkic radio field devices and wireless adapters, the communication, for example with a superordinated unit, is conducted as a rule via the wireless interface of the radio field device or the wireless adapter. Additionally, such radio field devices or wireless adapters have, as a rule, a hardwired communication interface. For example, in the HART® standard, it is provided that radio field devices must also have a hardwired communication interface, in addition to a wireless interface. Via such a hardwired communication interface, for example, on-site configuration of the radio field device or the wireless adapter is possible via a service unit, such as, for example, a handheld communicator connected to the hardwired communication interface. Additionally, the hardwired communication interface can be embodied as a fieldbus communication interface, so that the communication is conducted thereacross according to a bus system, such as, for example, according to one of the standardized bus systems, Profibus®, Foundation® Fieldbus or HART®. Via such a fieldbus communication interface, the radio field device or the wireless adapter can also be connected to a corresponding hardwired fieldbus.
The energy supply unit or the electrical current source of a wireless adapter or a radio field device is, for example, a disposable battery provided in the wireless adapter or the radio field device, a fuel cell, a solar energy supply, and/or a rechargeable battery.
If field devices or radio adapters are fed from an energy supply unit with limited energy supply, problems regarding sufficient explosion protection can occur. Problems show themselves as soon as the field device or the radio adapter needs to be connected to a higher voltage source, or when assemblies present in the field device or in the radio adapter produce higher voltages than the energy supply unit. In this case, the voltage supplied part of the field device or radio adapter must have a barrier, which fulfills the following two tasks:                prevention of an electrical current flowing back to the energy supply unit        protection against wrong connections.        
In the explosion endangered region, supplementally, the following requirements must be fulfilled:                prevent spark formation in the case of disconnection of the energy supply unit        sealing-off internal charge stoners or voltage sources from the outside.        
A known solution for the above-mentioned problem provides a barrier of diodes connected in series. For example, through a series circuit of three diodes, the explosion protection type ex-ia can be implemented. The disadvantage of the known solution is to be seen in the fact that the voltage drop across the diodes leads to a relatively high power loss, which reflects negatively on the lifetime of the energy supply unit, especially the battery. The voltage drop becomes greater with increasing electrical current flowing from the battery.
Another known solution that solve parts of the above-mentioned tasks provides electronic circuits, which usually are integrated in a circuit and generally referred to as “ideal diodes”. In these circuits, the electrical current flow direction is ascertained and the electrical current, in the case of wrong flow direction, is interrupted by means of a switch, e.g. by means of an FET. The disadvantage of this method is the relatively long reaction time of the circuits: In the case of disconnecting the battery, there is the danger of spark formation, which can have catastrophic consequences in explosion-endangered regions. Through the too-slow reaction of these circuits, charge quantities greater than 40 μJ can also penetrate these barriers, which is not allowable in regions of explosion protected environments.